DVDs and CDs are used as a storage media for digital and digitized information. They are made from one or more component discs of optical grade polycarbonate. In the process of manufacturing an optical disc, a nickel stamper (a metal matrix that contains digital data in the form of data protrusions) is used to form an information carrying layer in the polycarbonate. Specifically, polycarbonate is injected into a mold holding a nickel stamper. A negative image of the protrusions is formed in the polycarbonate during the molding process. The molded image (which consists of data pits) is subsequently covered with a reflective coating. Then the reflective coating is sealed with a protective layer, for example, a lacquer.
Presently, stampers are manufactured utilizing an electro-forming or electro-plating galvanic process. The process creates circular stampers (approximately 140 mm outer diameter, 34 mm inner diameter) that are electrolytic, nickel substrates (about 300.+-.3 microns thick). During the process, metal ions dissolved in a nickel sulfamate solution are deposited over the electrically conductive surface of a glass master. The glass master is manufactured by a laser mastering process. Typically, glass masters are manufactured from highly-polished, circular glass pieces (for example, 240 mm in diameter, 6 mm thick) covered with a layer of photo-resist material. The mastering process involves laser beam recording or conversion of digital data into geometrically shaped data pits. Data pits are formed in the photo-resist layer covering the glass master's top surface. Subsequently, the photo-resist covered glass master surface is vacuum metalized (with silver, nickel, or other suitable materials) to make it electrically conductive. Once vacuum metalized, a glass master is ready for use in the electro-forming process of creating a nickel stamper.
The data pits in the glass master are precisely replicated in the electro-forming process as nickel ions are gradually deposited over the conductive surface of the glass master. Using present technology, this process takes approximately one hour. After the desired stamper thickness is achieved (determined by a current/time/deposition rate calculation according to Faraday's law), the glass master and stamper are removed from the electro-forming galvanic cell. Subsequently, the nickel stamper is separated from the surface of the glass master. The nickel stamper is a negative copy of the glass master.
Stampers produced in known processes usually have a non-uniform thickness. Stamper thickness non-uniformity is caused by non-uniform current distribution of nickel-carrying electrolyte. Non-uniform current distribution also increases the time required for stamper manufacturing. One method of reducing these problems is to reduce nickel deposition rates (via lower current density) and increase the distance between the anode (negative potential connected to a titanium wire basket containing nickel pellets) and the cathode (positive potential connected to a metalized glass master) surfaces. However, even these techniques do not produce a stamper with suitable characteristics in as short a period of time as is desirable.
Most presently used electro-forming systems are rotary-cathode systems based on the stampers utilized in vinyl record manufacturing. Rotary electro-forming systems consist of a large capacity tank or sump filled with a temperature and pH regulated nickel-sulfamate solution (approximately 60 gallons per single anode/cathode arrangement), a rotary cathode (negative potential, rotational speed up to 100 rpm), and a stationary anode (positive potential, titanium mesh basket with sulfur containing nickel pellets). When electrical potential is applied between the cathode and anode, the nickel pellets in the titanium mesh basket of the anode actively dissolve to the ionic state and "attach" or plate out on the cathode surface. The nickel deposition rate can be increased by increasing the rotation of the cathode; increased rotation decreases the ion diffusion zone at the cathode surface. Rotation also improves electrical current distribution and nickel deposit uniformity.
Rotary-cathode, electro-forming systems operate with a DC power supply of 0-250 amps and 0-24 volts. Higher DC power level or current density can be utilized (up to 1000 amps/ft.sup.2 for a given nickel sulfamate concentration), but at such levels electrical field uniformity and distribution are poor. Thus, in present systems current density is limited to about 150 amps/ft.sup.2. This relatively low current density reduces overall process throughput (the amount of time required for stamper manufacturing).
Other rotary systems, including rotary-anode, electro-forming systems, are also available. These systems contain a rotary anode (with nickel pellets and rotational speeds of about 50 to 70 rpm) and an electrolyte pumping assembly. Pressurized electrolyte solution (at about 30 to 50 psig) is pumped through the anode housing at a flow rate of 10 to 12 gallons per minute. The electrolyte jets improve electrical field distribution (via improved ionic concentration) which, in turn, increases current density and the nickel deposition rate. However, even rotary anode systems are not completely satisfactory; Both rotary cathode and anode systems require rotational electrical contacts and high-power, gear motor drives. These requirements reduce overall system reliability and efficiency.
In response to the problems associated with rotary systems, there have been some attempts to design suitable stationary electro-forming systems (non-rotating anode and cathode). A typical stationary electro-forming system requires pressurized electrolyte solution to be pumped through its anode assembly directed towards or away from its cathode assembly. Rapidly pumped electrolyte solution provides for good electrolyte mixing and current distribution, which increases the nickel deposition rate. Further, the overall cost of a stationary system is lower than a rotary system. However, process reliability and quality control in present stationary electro-forming systems are lower than for presently available rotary systems.
The demands for faster, less expensive optical discs and increased data density of such discs necessitate improved quality and faster electro-forming systems (both for pre-recorded and recordable media). As described above, the disadvantages of stationary and rotary electro-forming systems, such as non-uniform nickel deposits and lengthy and complicated stamper manufacturing processing, are unacceptable. Accordingly, there is a need for an improved electro-forming system for producing nickel stampers.